Langmuir turbulence under Hurricane Gustav

Extreme winds and complex wave fields drive upper ocean turbulence in tropical cyclone conditions. Motivated by Lagrangian float observations of bulk vertical velocity variance (VVV) under Hurricane Gustav, upper ocean turbulence is investigated based on large eddy simulation (LES) of the wave-averaged Navier-Stokes equations. To realistically capture wind and wave-driven Langmuir turbulence (LT), the LES model imposes the Stokes drift vector from spectral wave simulations; both, the LES and the wave model are forced by the NOAA HRD surface wind analysis product. Results strongly suggest that without LT effects, simulated VVV underestimates the observed VVV. LT increases the VVV indicating that it plays a significant role in upper ocean turbulence dynamics. Consistent with observations, the LES predicts a suppression of VVV near the hurricane eye due to wind-wave misalignment. However, this decrease is weaker and of shorter duration than that observed, potentially due to large scale horizontal advection not present in the LES. LES results agree better with observations for smaller wind stresses, suggesting that the air-sea drag coefficient is lower than previously estimated in high wind tropical cyclone conditions. Both observations and simulations are consistent with a highly variable upper ocean turbulence field beneath tropical cyclone cores. Bulk VVV, a TKE budget analysis, and anisotropy coefficient (ratio of horizontal to vertical velocity variances) profiles all indicate that LT is suppressed to levels closer to that of shear turbulence due to misaligned wind and wave fields. VVV approximately scales with the directional surface layer Langmuir number. Such a scaling provides guidance for the development of an upper ocean boundary layer parameterization that explicitly depends on sea state.